Introduction
Facilitated diffusion also known as passive mediated transport or facilitated transport is specific passive transport of molecules or ions along a biological membrane. It is spontaneous process in which transport of ion occurs in a specific transmembrane integral proteins. Although it is a passive facilitated transport but it does not require the direct chemical energy from the hydrolysis of ATP in the transport step, rather, ions and molecules move down their concentration gradient to show its diffusive nature.
Facilitated diffusion is different from free diffusion in several ways:-
1. In facilitated diffusion transport occurs due to the molecular bindingbetween the cargo and the membrane embedded channels or carrier proteins.…show more content… Facilitator is an integral membrane protein that spans the membrane width. The force that is applied to drive the molecule of ion from one side of the membrane to the other is known as diffusion pores.
In the figure given below potassium ions are passing through the membrane with the help of potassium transport proteins. The ions are moving down a concentration gradient, thus, diffusion of potassium ions is taking place.
How does facilitated diffusion work
• Concentration gradient exists that allow the ions to diffuse into the cell without using cellular energy.
• Facilitate transport proteins shield these ions from the repulsive force of plasma membrane and they get diffused into the cell.
• Glycoproteins are present externally on the surface of plasma membrane. The ions or molecules that are in transported firstly get attached to the glycoprotein. This helps in the removal of material from the extracellular fluid which is required by the cell.
• Then the molecules are passed to the specific integral protein that facilitate their passage.
• Aquaporins are the channel proteins that allow the passing of water through the membrane at a very high…show more content… Glucose channels
• Glucose transporter, an example of carrier protein was identified in human RBCs.
• Kinetic studies indicate that the function of transporter protein occurs with the alteration between the two conformational states.
• In the first confirmation, the glucose binding site faces outside of the cell. Glucose binds to the exterior site which results in the confirmational change of transporter such that the glucose binding site now faces the interior region of the cell.
• Then the glucose is released into the cytosol and the transporter regains its original confirmation.
• Because the conformational changes of the glucose transporter can be reversed, so the glucose can be transported in opposite direction by reversing the step.
• This reverse flow takes place in liver cells where glucose is synthesized and released into circulation
Ion channels
Channel proteins forms pores in the membrane which opens to allow the entry of small molecules or polar molecules.
Porins
These are the cannel proteins that permit the free passage of ions and molecules through the outer membrane of bacteria. These ion channels may be gated and regulated or non-gated and always open or